1. Field of the Invention
The present invention relates to a method and device for waveform shaping of signal light.
2. Description of the Related Art
In an optical fiber communication system that has been put to practical use in recent years, a reduction in signal power due to transmission line loss, coupling loss, etc. is compensated by using an optical amplifier such as an erbium doped fiber amplifier (EDFA). The optical amplifier is an analog amplifier, which functions to linearly amplify a signal. In this kind of optical amplifier, amplified spontaneous emission (ASE) noise generated in association with the amplification is added to cause a reduction in signal-to-noise ratio (S/N ratio), so that the number of repeaters is limited to result in the limit of a transmission distance. Further, waveform degradation due to the chromatic dispersion owned by an optical fiber and the nonlinear optical effects in the fiber is another cause of the transmission limit. To break down such a limit, a regenerative repeater for digitally processing a signal is required, and it is desirable to realize such a regenerative repeater. In particular, an all-optical regenerative repeater capable of performing all kinds of signal processing in optical level is important in realizing a transparent operation independent of the bit rate, pulse shape, etc. of a signal.
The functions required for the all optical regenerative repeater are amplitude restoration or reamplification, waveform shaping or reshaping, and timing restoration or retiming. These functions are referred to as 3R functions, and in particular, the first and second functions are referred to as 2R functions.
An opto-electro-optic (O-E-O) conversion type device and an all-optical type device are known as a device for regenerating an optical signal on the optical level. The O-E-O conversion type device is a device for once converting an optical signal into an electrical signal, next regenerating the electrical signal on the electrical stage, and finally modulating laser light by using the regenerated electrical signal to convert it into an optical signal. On the other hand, the all-optical type device is a device for regenerating an optical signal optically without conversion into an electrical signal. As the O-E-O conversion type device, a device operating at about 10 Gb/s has already been put to practical use. However, since the performance of this device depends on the operating speed of electronics, it is difficult to operate the O-E-O conversion type device at 40 Gb/s or more. On the other hand, the all-optical type device has not yet reached a practical level, but some methods have been proposed.
A Mach-Zehnder interferometer (MZI) type optical gate is known as a typical all-optical waveform shaping device. This optical gate is configured by integrating a Mach-Zehnder interferometer including first and second nonlinear optical media each for providing a phase shift on an optical waveguide substrate, for example. Probe light as continuous wave (CW) light or optical pulses is equally divided into two components, which are in turn supplied to the first and second nonlinear optical media. The optical path length of the interferometer is set so that output light is not obtained by interference of the two components of the probe light.
An optical signal is further supplied to one of the first and second nonlinear optical media. By properly setting the powers of the optical signal and the probe light, a converted optical signal synchronous with the optical signal is output from the optical gate. The converted optical signal has the same wavelength as that of the probe light.
It has been proposed to use a semiconductor optical amplifier (SOA) as each of the first and second nonlinear optical media. For example, an InGaAs SOA having opposite end faces treated with antireflection coatings is used as each nonlinear optical medium in a 1.5 μm hand, and these nonlinear optical media are integrated on an InP/GaInAsP substrate to fabricate an optical gate. In particular, a type of optical gate called an ultrafast nonlinear interferometer (UNI) has been proposed to improve an operating speed. In this interferometer, the timing of passing of optical pulses through one of the two optical paths is slightly shifted from the timing of passing of optical pulses through the other optical path to thereby improve the limit to the operating speed due to the carrier effect of a semiconductor medium.
A nonlinear optical loop mirror (NOLM) is known as another conventional waveform shaping device. The NOLM includes a first optical coupler having first and second optical paths directionally coupled to each other, a loop optical path for connecting the first and second optical paths, and a second optical coupler having a third optical path directionally coupled to the loop optical path.
By forming a part or the whole of the loop optical path from a nonlinear optical medium and supplying probe light and an optical signal respectively to the first optical path and the third optical path, a converted optical signal is output from the second optical path.
An optical fiber is generally used as the nonlinear optical medium in the NOLM. In particular, a NOLM using a SOA as the nonlinear optical medium is referred to as an SLALOM (Semiconductor Laser Amplifier in a Loop Mirror).
By combining a waveform shaping device and an optical clock regenerating (recovery) device, the 3R functions can be provided. In this case, generally, an optical signal to be 3R-regenerated and regenerated optical clock pulses are input into an optical AND circuit, in which data owned by the optical signal is transferred to the regenerated optical clock by the AND operation. The waveform shaping can be effected by locating the waveform shaping device on the front or rear stage of the optical AND circuit, or by making the optical AND circuit have a waveform shaping function.
The 3R functions are conventionally provided by an MZI type optical gate switch as mentioned above, for example. The present inventors have already proposed a waveform shaping device for providing the 2R functions and/or the 3R functions (Japanese Patent Application No. Hei 11-293189). In this device, two NOLMs are combined to thereby increase the degree of freedom of wavelength conversion in the case of obtaining a function of waveform shaping or optical gate.
The waveform shaping device for providing the optical 3R functions has a problem that sufficient 3R functions cannot be obtained in some case according to a relative temporal deviation between the optical signal and the regenerated optical clock pulses input into the optical AND circuit. Such a relative temporal deviation may be caused by the walk-off between the optical signal and the regenerated optical clock pulses in the optical AND circuit or the temporal instability and jitter of each optical pulse, for example.